Natural Radioactivity in the Geologic Environment

Bruce W. HurleyNational Nuclear Security Administration

Nevada Site OfficeCEMP

July 28, 2009

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Most Abundant Radioactive Elements

• Potassium, isotope 40 (40K)

• Uranium, isotopes 238 & 235 (238U & 235U)

• Thorium, isotope 232 (232Th)

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238U α> 234Th β> 234Pa β> 234U α

> 230Th α

> 226Ra---------------------------------------------------------------------------------226Ra α

> 222Rn α

> 218Po α

> 214Pb β

> 214Bi

---------------------------------------------------------------------------------214Bi β

> 214Po α

> 210Pb β

> 210Bi β

> 210Po α

> 206Pb

Not shown: 14 gamma emissions from 226Ra, 214Pb, & 214Bi

Uranium 238 Decay Products

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• Potassium, uranium and thorium form large positive ions (cations). These “fat cats” require minerals with large crystal structure openings to fit into.

• Such minerals are most common in certain types of rocks “evolved” through processes in the Earth’s crust. These rocks may have evolved by either the “distillation” of molten magma or “sifting” by ‘sedimentation.

Lookin’ for a home…

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• The rock which commonly contains the highest radioactive element content is granite. Granite evolves by the concentration of light weight minerals from magmas generated deep within the crust. These “lightweights” are generally light in color and contain high concentrations of elements which do not fit into dense mineral structures.

Granite

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• 40K is most commonly concentrated in feldspars (K-spar family) & micas, while uranium and thorium are incorporated in such accessory minerals as zircon & sphene.

• Radioactive minerals often leave tell-tale “baked” zones or “halos” in surrounding non- radioactive minerals.

Radioactivity in Granite

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• Granite forms from the slow cooling of magma deep beneath the Earth’s surface. If the magma erupts onto the surface, it forms either rhyolite lava flows or deposits of ash fall or flow tuffs. Erupted granitic rocks are very fine-grained, and often contain the radioactive elements distributed within very fine volcanic glass particles. Granite, rhyolite, and tuff all have similar radioactive element contents.

Granite Rocks

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• One type of granitic rock in which radioactive minerals are particularly common is the very coarse-grained variety known as pegmatite.

• Pegmatite usually contains rare earth element minerals as monazite, which host U & Th. In pegmatite, U & Th also sometimes form their own minerals, such as uraninite (UO2 ) & thorianite (ThSiO4 ).

Pegmatite

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• Shale is a sedimentary rock which evolves through the weathering of rocks by chemical decomposition & water transport, deposition, & compaction of mud. Most of the minerals in shale are clay minerals, which have large openings in their crystal structures. K, U, & Th fit well into clays & their cousins, the zeolites. K is also present in very small particles of mica & feldspar.

Shale

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• Sandstone & conglomerate are sedimentary products of weathering, erosion, deposition & cementation, similar to shale. Unlike shale, they seldom have a high content of radioactive minerals when deposited. However, they are much more porous and permeable & often host U & Th minerals deposited by groundwater. Many major uranium mines are in sandstone and/or conglomerate.

Sandstone and Conglomerate

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• Bedded phosphate or phosphorite deposits form when phosphorus from seawater is incorporated in the shells of tiny marine organisms as the calcium phosphate mineral apatite, and deposited as a biochemical sediment. Apatite hosts both U & Th in its structure. Likewise, the hydrocarbon compounds in coal may also host these elements. In some instances, both phosphate & coal contain enough uranium to be considered low-grade uranium ore deposits.

Phosphorite and Coal

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• Metamorphic rocks are formed by the effects of great heat & pressure on pre-existing igneous (magmatic) or sedimentary rocks. Metamorphic rocks formed from rocks which already contain a high content of radioactive elements tend to retain that content.

Metamorphic Rocks

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Some Typical K, U, Th-Bearing Minerals

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Relative Original Occurrence ofRadioactive Elements in Rocks

OccasionalAndesite

ConglomerateSandstone

Slate>GneissMetaconglomerate

RareBasalt

LimestoneBedded Gypsum/Salt

QuartziteMarble

CommonGraniteShale

Bedded PhosphateCoal

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• Radioactive minerals may occur either as residual or secondary deposits in soils.

• Residual deposits form because of the heavy weight of minerals bearing U & Th. Like gold, they tend to concentrate with coarse gravel in streams, and in black sands on beaches.

Radioactive Elements in Soils

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• Secondary deposits form due to the leaching of uranium from minerals during chemical weathering. Uranium is mobile in groundwater, but is readily precipitated by humic acids in soils. It may be found in carbonized plant or animal remains, and may also fill openings in porous soils.

Radioactive Elements in Soils(continued)

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• With the exception of some rare pegmatites, most ore deposits of U & Th are formed by weathering (leaching), transport, & deposition of these elements by groundwater. These deposits may occur as veins, pods, or blanket-like zones in a variety of host rocks, many of which originally contained little or no radioactive element content.

U and Th Ore Deposits